Orally-delivered phencyclidine (PCP) will be estblished as a reinforcer for twelve rhesus monkeys. Substitution experiments will be conducted to determine whether drugs that share discriminative stimulus properties with PCP substitute more readily for PCP as reinforcers than those that do not, to examine the variable of drug history in establishing drugs as reinforcers and to compare relative reinforcing efficacy of drugs within the same class. A second area of investigation concerns the variable of food deprivation which has recently been shown to produce marked increases in drug self - administration. A concentration-effect function will be compared under food satiation and deprivation conditions to establish whether food deprivation interacts with drug concentration, and blood levels of phencyclidine will be compared to determine if changes in responding under different feeding conditions are due to dispositional factors. Also, a hypothesis will be tested that food deprivation is an example of a larger phenomenon in which deprivation of one reinforcing substance results in increased responding for another reinforcing substance. The monkeys will be deprived of a preferred saccharin solution. A third major objective of the proposed research is to implement the use of second-order schedules with the oral drug self-administration model. Secon-order schedule performance will be used to evaluated the role of conditioned reinforcing stimuli (e.g., auditory, tactile, taste and visual) in maintaining high rates and long sequences of behavior leading to drug access. Second-order schedules, with drug access occurring at the end of the session, will also be used to determine whether food deprivation selectively increased drug intake, response rate, the reinforcing efficacy of the event maintaineing the behavior, or a combination of factors. In addition, the effect of training history (under a second-order schedule) will be measured on subsequent performance under a simple schedule. Finally, oral phencyclidine self-administration procedures will be used to investigate cross tolerance between phencyclidine and several of its analogs, to characterize variables leading to physical dependence, and to study interactions between PCP and other drugs.